The present invention relates to a scanning optical ranging system. It finds particular application in conjunction with a light detection and ranging (LIDAR) system and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.
A light detection and ranging (LIDAR) system transmits a pulse of light (e.g., from a laser), which is reflected from a target. An optical receiver detects the reflected light, and the range to the target is computed from the delay time between the transmission of the light pulse and the detection of the reflected light. The receiver field-of-view and the transmitted light beam are usually matched and co-aligned to ensure maximum light collection efficiency. If the LIDAR contains a fast optical scanner (such as a rapidly moving mirror), it is possible for the field-of-view of the receiver to lose alignment with respect to the projected light beam. Such alignment loss is caused by a change in the pointing direction that occurs during the time required for the light pulse to travel to the target, reflect, and then travel back to the receiver. The extent of this misalignment is a “lag angle,” which depends on the speed of the scanner and the range of the target. For a scanning LIDAR that is “diffraction-limited” (i.e., the light beam divergence is limited only by the wavelength and the diameter of the beam at the exit aperture of the LIDAR), the scanner angular speed ω1/2 (measured in radians/second) for which the lag angle is half of the transmitted light beam divergence is:
            ω              1        /        2              =                  θ        .            =                        0.5          ·          λ          ·          c                          2          ·          D          ·          R                      ,
where λ is the wavelength of the transmitted light (in meters),                c is the speed of light (˜3×108 meters/second),        D is the transmitted light beam diameter at the exit aperture of the LIDAR (in meters), and        R is the range to the target (in meters).        
If the receiver field-of-view is initially aligned to a transmitted light cone of illumination, the received signal is reduced by the lag angle. The effect becomes worse at longer ranges and as the scanner speed increases.
If the LIDAR system is not diffraction-limited and the transmitted beam divergence is instead Δ, then the formula above becomes:
      ω          1      /      2        =            θ      .        =                            0.5          ·          Δ          ·          c                          2          ·          R                    .      For example, if the divergence of the transmitted light beam is 2 milliradians (mrad) and the range of a target is 3 km, the angular speed at which the lag angle is half of the transmitter beam width is 50 radians/second, or 480 revolutions/minute (rpm). In this case if the receiver field-of-view is matched to the transmitted light beam divergence, the lag angle is still sufficiently small for the optical receiver to detect some reduced amount of scattered light from the target, but for scanner speeds greater than 960 rpm, the lag angle effect causes the receiver field-of-view to completely miss or obscure the signal from 3 km and beyond. The condition for this complete obscuration is:
      ω    obscure    =            θ      .        =                            Δ          ·          c                          2          ·          R                    .      
All LIDAR systems, whether they are scanning or staring, must often contend with another issue—large signal dynamic range. Signal dynamic range is the ratio of the maximum detectable light signal intensity (i.e., detector saturation) to the minimum detectable light signal intensity. The detected signal decreases rapidly with increasing target distance. Therefore, the received signals from targets at closer ranges may over-saturate the detector, while those from targets at longer ranges may be barely detectable. A design technique known as “geometric compression” can reduce the signal dynamic range by controlling the fixed overlap of the transmitter and receiver optical fields-of-view, the separation of the receiver and transmitter optics, and the shadowing of the receiver by the transmitter optics to attenuate the close-range signal. The time delay of the received light signal with respect to the transmitted light pulse does not enter into this compression calculation since these design parameters are static. Geometric compression can benefit both scanning and staring LIDAR systems.
The present invention provides a new and improved apparatus and method which addresses the above-referenced problems.